1. Field of the Invention
The present invention relates to a vertical MOSFET (insulated gate field effect transistor), a method of manufacturing the vertical MOSFET, and a complex semiconductor device.
2. Description of the Background Art
As a high breakdown voltage switching element used for an inverter circuit 1000 shown in FIG. 6, an IGBT 101 is widely used. The IGBT 101, having features of a bipolar transistor, such as a high breakdown voltage and a low on voltage, and the superior feature of being able to operate at high speed although being lower in speed than an MOSFET, is an important semiconductor element which underpins the present power electronics.
However, the IGBT 101 shown in a main portion sectional view of FIG. 7A, as it has a reverse breakdown voltage junction (a collector junction 103), unlike an MOSFET 301 shown in FIG. 7B, usually cannot cause current to flow in a reverse direction (a bias direction in which an emitter E is positive and a collector C is negative). When the IGBT 101 changes from a conduction state to a forward blocking state, it may happen that a high surge voltage is generated in the reverse direction due to an inductance component in the circuit. There is fear that the IGBT 101 which is not protected from a reverse breakdown voltage usually breaks down when the surge voltage is applied to the IGBT 101, but when the IGBT 101 is used in the inverter circuit, the IGBT 101 is protected by a diode 401 (FIG. 6) connected in reverse parallel to the IGBT 101 in order to reflux an L load current generated every time the IGBT 101 is turned off.
As the parallel connection of the standard flux diode 401 to the previously described kind of IGBT 101 has a limitation in increasing the speed of switching, it has recently been studied, in response to a growing demand for an increase in the frequency of an inverter, to replace the IGBT 101 with a super junction MOSFET 201 shown in FIG. 5A. The super junction MOSFET 201 (FIGS. 5A and 5B), with which it has been studied to replace the IGBT 101, has in a drift layer 205 a parallel pn layer 202 of a super junction structure wherein a plurality of parallel pn junctions are narrowly spaced in a direction perpendicular to the principal surface. The inner region of the parallel pn layer 202 is formed of n-type drift regions 202a and p-type partition regions 202b. As the super junction MOSFET 201 is such that it is possible to deplete all the parallel pn layer 202 at a low voltage by narrowing the pitch width of the parallel pn layer 202 even when the concentration of the n-type drift regions 202a in the parallel pn layer 202 is made higher than a standard impurity concentration compatible with a breakdown voltage, the super junction MOSFET 201, despite being of a unipolar type, has the feature of being and high in breakdown voltage and low in on resistance. Furthermore, the super junction MOSFET 201, as it is capable of high speed switching deriving from a unipolar device, as well as having a built-in reverse diode structure (reference signs 203 and 202a in FIG. 5A), also has the advantage of it being possible to hope for a reduction in the size of the device without a need to newly connect the parallel diode 401 of the inverter circuit of FIG. 6.
In the super junction MOSFET 201, the carrier lifetime during the reverse recovery of the built-in diode, when not being controlled, is constant in a depth direction from the front surface of a substrate, as shown in FIG. 5B.
A document relating to this kind of super junction MOSFET is disclosed in which it is described that a super junction (hereafter SJ) structure formed of a parallel pn layer, and an n-type buffer layer below the SJ structure, the impurity concentration of which is changed in two stages, are provided in the drift layer 205, thereby reducing on resistance and forming the reverse recovery characteristics of a built-in diode into a soft recovery waveform (JP-A-2003-101022 (FIG. 11 and Paragraphs 0077 to 0079)). A semiconductor device including a super junction MOS structure wherein a reverse recovery time is shortened without increasing a drain-source leak current is already known (Domestic Re-publication of PCT Application 2010-24433 (Abstract)). Also, it is described that an SJ-MOSFET is connected to a schottky barrier diode including the SJ structure, thereby enabling a semiconductor device suitable for a soft switching system (JP-A-2006-24690 (Problem and Solution in Abstract)). It is shown that a lifetime control region is provided in the whole of the schottky barrier diode including the SJ structure, thus reducing a reverse current and improving reverse recovery characteristics (JP-A-2008-258313 (Abstract)). A description is given of a lifetime control method for forming reverse recovery characteristics into a soft recovery waveform (JP-A-2007-59801 (Abstract)). Various descriptions are given of an excess minority carrier lifetime control method (JP-A-7-226405 (Problem)). Furthermore, a description relating to a semiconductor device wherein it is possible to improve breakdown voltage and turn-off characteristics compared with a heretofore known element is disclosed (JP-A-2001-102577 (Problem)).
In the super junction MOSFET 201 shown in FIG. 5, when in a forward blocking state, a depletion layer expands all the way out in each column in the parallel pn layer at a low breakdown voltage and is completely depleted. At this time, the built-in diodes 203 and 202a transit from a state, in which a forward current (a reflux current) is flowing, to a reverse bias blocking state (that is, a reverse recovery state) of the pn junctions of the built-in diodes. However, the built-in diodes are such that, because of a unipolar structure, there are very few minority carriers, and a reverse recovery current Irp is small, and in addition, that it is easy to form a so-called hard recovery waveform wherein a current waveform and voltage waveform rise steeply. When a reverse recovery operation forms a hard recovery waveform, the problem is that ringing occurs (an oscillatory waveform is formed), thus causing noise generation, as shown in a reverse recovery waveform diagram of the heretofore known super junction MOSFET of FIG. 4 (in FIG. 4, the oscillatory waveform portions are overlapped in a blacked out condition and difficult to see). The waveform of the heretofore known structure of FIG. 4 is a result from simulating a reverse recovery operation current waveform with a supply voltage set to be 400V, a forward current set to be 20 A, and a temporal change in reverse current set to be 100 A/μs, for the vertical super junction MOSFET of the heretofore known structure shown in FIG. 5A.